Hi, I know I am literally new and this is my first post. I had this same issue with a Mac Mini and this is what I did that easily fixed this issue.
Note: I am saying what worked for me, results may vary.
Please read all instructions before you start, because they might not be in order.
1. Open activity monitor and you will see that Agent that's causing us trouble. (Don't quit or force quit it yet)
2. Run your Office Update to the point where that agent is holding us back.
You have to be fast on the next step.
3. As soon as Office requests the closure of the agent ("Closing Application"), go to your activity monitor and quit that wicked agent.
If your Apple device is password protected, your will get that prompt to enter your admin password and there goes your successful installation.
I hope this helps someone out there, and Let the odds be in your favor!